Selecting low frequency antigen-specific single B lymphocytes

ABSTRACT

Disclosed are immunofluorescence staining methods which increase the likelihood that antibodies expressed by a single B cell selected and sorted by fluorescence activated cell sorting are specific for the antigen of interest, and which also allow selection of B cells expressing antibodies of high affinity for the antigen of interest. The selection for B cells expressing antibodies to specific antigens is increased by labeling B cells with at least two antigen probes, where each antigen probe includes the antigen of interest and is labeled with a different fluorochrome. The positive selection is preferably combined with a negative selection step, in which autofluorescent cells and sticky cells are excluded out. The specificity of sorting of the desired B cells can be further enhanced by staining those antigen-specific B cells which produce the immunoglobulin isotype (typically IgG), with targeting molecules reactive with a B cell marker, such as γ chain and CD19, that are conjugated with a different fluorochrome. Thus, the antigen-specific IgG-producing B cells of interest may be labeled with these unique reagents in four color FACS (including one for negative selection), which can sort the desired antigen-specific B cells at enhanced proportions. After sorting the B cells with FACS, those B cells with high affinity are preferred for analysis by the single cell-PCR procedure to amplify and clone the V H  and V L  segments of interest.

RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 07/905,040, filedJun. 26, 1992, (now U.S. Pat. No. 5,256,542), which is acontinuation-in-part of Ser. No. 07/848,249, filed on Mar. 9, 1992 (nowU.S. Pat. No. 5,213,960).

FIELD OF THE INVENTION

The invention relates to the use of a unique cell staining methodologyin conjunction with multiple-color fluorescence-activated cell sortingto identify and isolate low-frequency B cells which express monoclonalantibodies with a particular antigen binding specificity and,preferably, a high affinity, for the purpose of amplifying the V_(H) andV_(L) sequences in those individual B cells.

BACKGROUND OF THE INVENTION

B cells produce antibodies, and each B cell produces an antibody havingone particular antigen specificity (i.e., the antibody is monospecific).If one expands a single B cell clone, the antibodies produced by theexpanded population of B cells are homogeneous. Hybridoma technology,which has been in existence since 1975, enables one to immortalizeindividual B cells, and thereby allows one to expand a population ofindividual B cell clones to obtain sufficient antibodies so that theimmortalized population can be screened to isolate those B cellsproducing antibodies having a particular antigen specificity. The B cellhybrids of interest can be grown in a large scale to make largequantities of homogeneous monoclonal antibodies, which are useful fordiagnostic and therapeutic purposes. Kennett, R. H,. et al., MonoclonalAntibodies (Plenum Press, New York 1980); Borrebaeck, C. A. K. &Larrick, J. W., Therapeutic Monoclonal Antibodies (Stockton Press, NewYork 1990); Winter, G. & Milstein, C. Nature 349:293 (1991).

Hybridoma technology generally works best for preparing purely murinemonoclonal antibodies. Hybrids made from fusing human B cells with humanor murine myeloma cells are generally unstable, and tend to lose thehuman chromosomes and the ability to produce antibodies. Transforminghuman B cells with the Epstein-Barr virus (EBV) offers an alternativeway to immortalize them. However, the transformation frequency isrelatively low and the transformants are also not stable and often loseantibody-producing ability.

An alternative method, often referred to as the "Combinational V regionlibrary" method, has been developed to identify and isolate antibodyfragments having a particular antigen specificity. Sastry, L. et al.,Proc. Natl. Acad, Sci. U.S.A. 86:5728 (1989); Huse, W. D. et al.,Science 246:1275 (1989); Orlandi, R. et al., Proc. Natl. Acad. Sci.U.S.A. 86:3833 (1989). This technique relies on polymerase chainreaction (PCR) techniques. Using degenerate oligonucleotide 5'-endprimers corresponding to the leader peptides or the N-terminal frameworksegments of V_(H), V.sub.κ, and V.sub.λ, and 3'-end primerscorresponding to the CH1 domain of a heavy chain such as γ and to theconstant regions of light chains κ and λ, the V_(H), V.sub.κ, andV.sub.λ regions of a population of B cells are amplified with PCR. Thepools of these V_(H), V.sub.κ, and V.sub.λ genes are referred to asV_(H), V.sub.κ , and V.sub.λ libraries of a particular B cellpopulation. Using specially designed vectors derived from abacteriophage, a V_(H) segment and a V_(L) segment can be inserted intothe vector and coexpressed in an E. coli bacterial host cell. The randomcombination of the V_(H) and V_(L) libraries creates an extremely largeV_(H) -V_(L) combinatorial library. Each pair of combined V_(H) andV_(L) forms a unique antibody F_(V) fragment with a particularantigen-binding specificity.

The recombinant λ phage including the expression vectors that containthe combined V_(H) and V_(L) genes can be cloned, and then variousmethods can be applied to determine the antigen-reactivity of theantibodies which are expressed. In one earlier expression system, thecombined V_(H) and V_(L) gene segments in bacteriophage λ were expressedas soluble Fab fragments in E. coli. In a more recently developedsystem, the combined V_(H) and V_(L) fragments are expressed as a partof a protein on the surface of a filamentous bacteriophage, designatedfd. In this latter system, the bacteriophage expressing the antibodyfragment of interest can be isolated using affinity chromatography.McCafferty, J. Nature 348:522 (1990).

When using EBV transformation or cell fusion as described above toobtain monoclonal antibodies of particular specificity, a number offactors determine the probability of isolating the desired antibodies.Some of these factors are:

1. The pre-fusion number of B cells expressing the desired antibodies inthe B cell population (referred to as the frequency of desired B cells);

2. The number of B cells in the B cell population which are fused withmyelomas or transformed by EBV (respectively referred to as thefrequency of fusion or transformation);

3. The stability of the immortalized cells.

Referring to factor "1" above, the frequency of desired B cells in miceis affected by whether the mice from which the B cells are derived areunexposed, immunized, or hyperimmunized with the antigen. By someestimates, the antibody repertoire in a mouse could represent over 10¹⁰different antibody species. However, at any given time, there may beonly 10⁷ -10⁸ B cell clones in a particular mouse. Winter, G. &Milstein, C. Nature 349:293 (1991). This suggests that for mostantigenic epitopes, the frequency of B cells producing reactiveantibodies may be in the order of 1×10⁻⁵ or 1×10⁻⁶, or less. It is alsoknown that in a hyperimmunized mouse, the frequency of B cells specificfor the immunogen may account for several percent of the total, or more.

When making human monoclonal antibodies, the frequency of desired Bcells is affected by whether the human donors from whom the B cells arederived are immunized with the antigen through infection, vaccination,or other exposure. Depending on the intensity of a viral infection, thestage in the immune response, and the immunogenicity of the particularviral antigen, the frequency of B cells in the infected person specificfor a particular antigenic epitope on a viral protein may range from1×10⁻³ to 1×10⁻⁵ for the more dominant epitopes and less than 1×10⁻⁵ forthe relatively silent epitopes. The frequency of IgG-expressing B cellsspecific for a protein allergen will often vary among differentindividuals, and may vary with the concentration of the allergen in theenvironment. The frequencies of B cells specific for autologousantigens, such as IgG, platelet surface antigens, leukocyte surfaceantigens, nuclear proteins, or DNA, will also vary among differentindividuals. For a relatively nonimmunogenic antigenic epitope on alarge protein molecule, such as the CD4 binding site on gp120 of HIV-1,the B cells expressing antibodies specific for an antigenic epitopethereof in a moderately immunized mouse or an HIV-1 infected person mayonly occur at a frequency of 1×10⁻⁵ or lower among the IgG-bearing Bcells. For an antigen the B cell donor has not been exposed to, thefrequency of B cells specific for the antigen may be 1×10⁻⁶ or lower.

In a fusion procedure for preparing murine hybridomas, immunized mousespleen cells are fused with myeloma cells. The number of lymphocytes ina mouse spleen is about 1×10⁸. B cells account for about 10% of thetotal, and among B cells, IgG-expressing B cells account for about 20%.Thus, in a mouse spleen, there are about 2×10⁶ IgG-expressing B cells.

In performing a cell fusion or EBV transformation procedure toimmortalize IgG-expressing B cells from a person, one may typically take100 ml peripheral blood from the donor. There are about 1×10⁸lymphocytes in this amount of blood. Among them, B cells account forabout 1-1.5×10⁷, and IgG-expressing B cells account for about 2-3×10⁶.

If the targets of immortalization are antigen-specific IgG-expressing Bcells occurring at moderately low frequency, for example, 1×10⁻⁵ amongthe IgG-bearing B cells, there will only be about 20-30 such B cells ina mouse spleen or in 100 ml of peripheral human blood. The difficultywhich stems from having such a small number of B cells producing thedesired antibody relates to factor "2" above: the frequency of cellfusion or transformation. A skillful technician using a good fusionprotocol can generate about 10,000 hybrids from the approximately 1×10⁷B cells which are taken from the spleen of a mouse. This corresponds toa fusion frequency of 1 in 1000 B cells. The cell fusion andEBV-transformation frequencies for human B cells are at least about 1 to2 orders of magnitude lower. Thus, if one is starting with only about20-30 B cells with the desired antigenic specificity, the chances ofimmortalizing one of the desired B cells is very slim.

Factor "3" above refers to the stability of the immortalized cells. Asnoted above, while murine hybridomas are generally stable, humanhybridomas and EBV transformants are much less stable. Sequentialsubcloning is usually required to obtain stable human hybrids andtransformants. However, even with such subcloning, which can be alaborious procedure, stable antibody-secreting cell lines cannot beobtained for a large portion of human hybrids and transformants.

While the combinatorial library methodology can be used to produceantigen-specific antibody fragments in bacterial host cells, it alsosuffers certain drawbacks related to the low frequency ofantigen-specific B cells. For determining whether a particular B cellexpresses antibody molecules of desired antigen-binding specificity, theV_(H) and V_(L) segments of the mRNAs of the heavy and light chains needto be reverse transcribed into cDNA, amplified by PCR, and the resultinggene segments must be cloned, sequenced, and incorporated intoexpression vectors. The two gene segments must then be coexpressed in amammalian cell or other system, and the fragments produced must becharacterized for antigen-binding specificity and for relative bindingaffinity. In a natural immune response, the B cells expressing theantibodies specific for an antigen expand, resulting in higherrepresentations of such cells among the B cell repertoire. Thecombination of the desired V_(H) and V_(L) gene segments are expressedonly by the antigen specific B cells. The combinatorial librarymethodology separates the V_(H) and V_(L) genes and recombines them in arandom fashion. If B cells secreting an antibody of desiredantigen-binding specificity are present at a frequency of 1×10⁻⁵, thecombinatorial library will expand the V_(H) -V_(L) library. Depending onthe diversity of the repertoire in the V_(H) and V_(L) libraries, theoriginal V_(H) and V_(L) combination will be present at a frequency ofprobably about 1×10⁻¹⁰. It has been suggested that certain V_(H-) V_(L)combinations resulting from this genetic manipulation may also have thedesired antigen specificity. However, this possibility is yet to besubstantiated by experimentation. Winter, G. & Milstein, C. Nature349:293 (1991).

Screening large numbers of bacteriophage particles expressing thedesired fragment is generally more efficient than screening hybridomasor EBV transformants for the corresponding antibody. The typicalscreening methods for bacteriophage plaques can handle up to 10⁶ or 10⁷phages, assuming that the availability of antigen suitable for theantigen-binding assays is not a limiting factor. However, for certainsoluble antigens or antigenic epitopes, screening bacteriophagesincorporating the reactive antibodies is difficult, because the antigensare limited or only available in soluble form. Additionally, for certainother antigens including membrane-bound proteins, the conformationalepitopes on the antigens are altered if the antigens are solubilizedfrom the membrane, and they will not react with the fragments beingscreened.

Additionally, if one is screening the antibody-expressing bacteriophageson an immunoblotting plate, about 50,000 plaques can be screened on oneplate. In an extremely ambitious experiment, 100 plates may be screened.This screens about 5×10⁶ plaques or phages, which is much below thetheoretical number of plaques which need to be screened to obtain thedesired fragment.

A new method has been described for expressing the anitbody fragmentV_(H) -V_(L) as a part of a protein on the surface of filamentous phage.It has been claimed that the phage expressing the specific antibody canbe affinity purified from a large number of phages using anantigen-conjugated affinity column. McCafferty, J. et al., Nature348:552 (1990). Theoretically this sounds plausible, however, it is yetto be proven that the methodology can identify and purify one specificphage from 1×10¹⁰ phages. In addition, one major problem inimmunoblotting and affinity chromatography methods is that antibodieswith a moderate affinity for the antigen will be selected. This allowsthe inclusion of many cross-reactive or sticky antibodies, causingburdens in the sequential screening procedures.

Various methods have been employed to enrich the desired B cells whenthey occur at low frequency. These methods can be used to enrich thedesired B cells whether one is attempting to isolate the V_(H) and V_(L)fragments by hybridoma fusion technology, EBV transformation, or thecombinatorial library methodology. These enrichment methods includefluorescence-activated cell sorting (FACS), panning againstantigen-coated plastic surface, binding to antigen-coated magneticbeads, or rosetting with antigen-coated red blood cells.

These procedures for enriching antigen-specific B cells, however, allsuffer from a certain degree of nonspecificity. For example, panning orabsorbing to plates or beads yields from about one to several percent ofnonspecific binding. Rosetting of cells by antigen-coated red bloodcells also has about the same degree of nonspecific activity. For cellsorting using FACS, the nonspecific sorting will depend on thestringency of the gate setting, but it usually ranges from 0.1 toseveral percent depending on the nature of the antigen.

An additional problem in a typical FACS procedure where mixed leukocytesare screened is background noise. An FACS apparatus operates by having anumber of channels, which can each view a different fluorochrome. Singlecells can be selected based on their fluorescence, or lack offluorescence. Background noise in an FACS apparatus arises primarilyfrom two sources. One source of noise is exhibited by certain activatedcells or proliferating cells of various leukocyte subpopulations thatcontain high concentrations of certain metabolites, which cause thesecells to exhibit autofluorescence even in the absence of fluorochromelabeling. Another possible source of background noise arises becausesome of the activated cells, and some monocytes and macrophages, possessstickly celluar plasmamembranes. Fluorescent probes willnon-specifically adhere to the sticky plasmamembrane and create anadditional source of background fluorescence in the cell sample.

Assuming that the B cells expressing antibodies having the desiredantigen-binding properties in a human donor occur at a frequency ofabout 10⁻⁵ to 10⁻⁶ (which is a reasonable estimate for B cells specificfor a weak antigenic epitope in an individual with exposure to theantigen or an individual naive to the antigen) the B cell enrichmentprocedures discussed above are not useful. If an enrichment methodprovides 1% nonspecific activity, then the desired B cells will averageonly about one in 1000 or 10,000 B cells isolated. Obviously, the singlecell-PCR procedure to be described below for identifying and isolatingantibodies of desired antigen-binding specificity would not work at sucha low frequency. Thus, a method to increase the proportion ofantigen-specific B cells among the single B cells selected forperforming the single cell procedure is needed.

SUMMARY OF THE INVENTION

The invention includes methods that increase the likelihood thatantibodies expressed by a single B cell selected by afluorescence-sorting technique, FACS, are specific for the antigen ofinterest, and that also allow selection of B cells expressing antibodiesof high affinity for the antigen of interest. The sorting for B cellsexpressing antibodies to specific antigens is increased by labeling Bcells with at least two antigen probes, where each antigen probeincludes the antigen of interest and the difference between the twoprobes is that each is labeled with a different fluorochrome, F1 and F2,respectively. Preferably, the sorting of desired B cells is alsoincreased by excluding autofluorescent cells that exhibit fluorescenceat greater than a specified level, and by excluding sticky cells ofvarious leukocyte subpopulations that exhibit fluorescene forfluorochrome F3, when the mixture of cells are treated with F3conjugated to a probe specific for an irrelevant cell type, such as amonoclonal antibody specific for a T cell surface antigen. The positiveselection using two antigen probes will exclude those B cells expressingantibodies specific for either of the fluorochromes. The negativeselection achieved by gating out all cells exhibiting fluorescence atgreater than a specified level, or fluorescence to fluorochrome F3, willexclude not only cells expressing the marker the probe is specfic for,but also those cells, including B cells, which do not express such amarker but are autofluorescent or possess sticky membranes that any ofthe fluorescent probes adhere to non-specifically.

Also, to further increase the specificity of the selected B cells, it ispreferred if, when cross-linkers or carrier molecules are employed inthe conjugation of antigen to fluorochrome, different cross-linkers andcarrier molecules are used for each of the two antigen probes.

The specificity of sorting of the desired B cells can be furtherenhanced by labeling those B cells which produce the immunoglobulinisotype (typically IgG) of interest. The surface antigens suitable forlabeling are the γ chain, κ or ξ chains, CD19, Ia, and the Fc receptors.This labeling of antigens on the B cells is preferably done with onetargeting molecule, associated with fluorochrome F4. For example,affinity-purified IgG-F(ab')₂ of goat-anti-human IgG (γ chain) oraffinity purified IgG-F(ab')₂ of rabbit-anti-human CD19 may be used asthe specific probe for the targeted B cell population. Thus, theantigen-specific IgG-producing B cells of interest may be labeled withthese unique reagents in three or preferably four-color FACS (one colorfor negative selection), which can sort enhanced proportions of thedesired antigen-specific B cells by a combination of positive-selectionand negative-selection gatings.

The differences in relative intensities between the antigen labels andthe isotype labels (e.g., IgG labels) among the different antibodies ofthe single cells selected can be used to determine the relative antigenbinding affinity of those antibodies. For example, the ratio of antigenprobe label to IgG label (e.g., the fluorescence intensity F1/F4) can becalculated for each labeled B cell. The higher the ratio, the higheraffinity the antibodies on the B cells have for the antigen. Aftersorting the individual antigen-specific B cells with FACS into separatecontainers, such as wells of 8×12 (96 well) microculture plates, those Bcells with high F1/F4 ratios are preferred for analysis by the singlecell-PCR procedure to amplify and clone the V_(H) and V_(L) segments ofinterest. This selection technique can result in V_(H) /V_(L) fragmentswhich are highly specific for the antigen of interest.

It is preferred that antigen-specific antibodies on B cells withfrequencies of about 10⁻⁵ or lower are labeled in the manner described,because B cells expressing antibodies binding to the carrier proteins,fluorochromes, or cross-linkers may have comparable or even higherfrequencies. Accordingly, unless one is using the labeling system of theinvention, B cells and other cells with sticky membranes or whichautofluorescence, and B cells binding to carrier proteins,fluorochromes, or cross-linkers may become labeled along with thosewhich express antibodies to the desired antigen, and those cells willget sorted and selected along with those expressing antibodies to thedesired antigen.

It is also noted that the labeling and sorting of the labeled B cellscan be done at the same time, or sequentially. For example, the antigenprobes labeled with the different fluorochromes can be introduced intothe B cell population at the same time. Or, alternatively, one set ofantigen probes (all labeled with the same fluorochrome) can beintroduced, and then these can be sorted out using the FACS method, andthen another set of antigen probes (labeled with a second fluorochrome)can be introduced into the population of sorted cells. The population ofsorted cells is then sorted to determine which are labeled with thesecond antigen probe.

The invention is described in further detail below.

DETAILED DESCRIPTION OF THE INVENTION A. Rationale of selecting andisolating low-frequency antigen-specific B cells for single cell PCR

For immortalizing antigen-specific B cells that occur at lower than1×10⁻⁵ in frequency and in fewer than 1000 cells in a sample, theexisting hybridoma methods, EBV-transformation methods, andcombinatorial V region library methods are deficient. The methods of thepresent invention are designed to select and isolate theantigen-specific B cells of interest preferably for use in V_(H) andV_(L) PCR analyses and production of antibody fragments.

The present invention relies on two techniques which have both beenestablished for working with single cells. One technique incorporates aspecial accessory which can be used in FACS to deflect single cells intoseparate containers. Both FACS Star Plus™ from Becton Dickinson (FosterCity, CA) and Epics C from Coulter Epics Division (Hialeah, FL) haveoutfitted cloning accessory apparatus which can be programmed todispense single particles or cells into selected compartments of astandard 96 well microtiter culture plate. This kind of cell sortingaccessory has been employed to subclone hybridoma cells which secretehigh amounts of immunoglobulins. Marder, P. et al., Cytometry 11:498(1990).

The other technique relied on is that PCR can be performed on single Bcells to amplify the V_(H) and V_(L) segments. Larrick, J. W. et al.,Bio/Technology 7:934 (1989). Numerous studies have indicated thatdegenerate oligonucleotides can be prepared to serve as the 5'-endprimers for V_(H) and V.sub.κ or V.sub.λ. The combinatorial librarymethod of making targeting molecules relies on such primers.Furthermore, numerous experiments have shown that PCR can amplify thegene segments of interest, such as V_(H) and V_(L), from a single cell.Because of the ability to work with even a single cell, the PCR approachis preferred in the present invention for producing targeting moleculeswhere the B cells of interest occur at low frequency.

Oligonucleotides suitable for use as the 5'-end primers for human V_(H)and V.sub.κ have been shown to amplify these segments almost 100% of thetime in single cells taken from the human lymphoblastoid IgG cell lines.The preferred set of 5'-end primers for V_(H) consists of 5 degenerategroups of oligonucleotides and one nondegenerate oligonucleotide (SEQ IDNOS:1-6), totaling 53 sequences. This set of primers corresponds to thefirst 5 amino acids of the mature immunoglobulins, and covers the knownsequences of the 155 human V_(H) segments which have been sequenced.

Referring to SEQ ID NO:1, several other oligonucleotides based on it arealso within the set of 5'-end primers for human V_(H). These otheroligonucleotides are variations on SEQ ID NOS:1-2 and SEQ ID NOS:4-6.The variations in SEQ ID NO:1 which yield these other oligonucleotidesis that the first Guanidine nucleotide can be a Cytosine, the sixthGuanidine can be a Thymidine, and the thirteenth Guanidine can be aThymidine.

The variations in SEQ ID NO:2 which yield these other oligonucleotidesare that the fourth, eighth and ninth Adenosines can be a Guanidines.The variations in SEQ ID NO:4 which yield these other oligonucleotidesare that the sixth Guanidine can be an Adenosine or a Cytosine, and theninth and twelfth Adenosines can be Guanidines. The variations in SEQ IDNO:5 which yield these other oligonucleotides are that the fourthAdenosine can be a Guanidine, the eighth Adenosine can be a Cytidine,and the twelfth Adenosine can be a Guanidine. The variations in SEQ IDNO:5 which yield these other oligonucleotides are that the fourthGuanidine can be a Thymidine, the tenth Cytidine can be a Thymidine, andthe fourteenth Cytidine can be a Guanidine. The variations in SEQ IDNO:5 which yield these other oligonucleotides are that the fourthGuanidine can be a Thymidine, the tenth Cytidine can be a Thymidine, andthe fourteenth Cytidine can be Guanidine. These variations in SEQ IDNOS:1-2 and 4-6 will yield the 53 different oligonucleotides whichconstitute the set of 5'-end primers for V_(H).

The preferred set of 5'-end V_(K) primers consist of 2 degenerate groups(SEQ ID NOS:9 and 12) and 4 nondegenerate oligonucleotides (SEQ IDNOS:7-8 and 10-11), totaling 10 sequences. The variations in SEQ ID NO:9which yield other oligonucleotides are that the third Cytidine can be aThymidine, and the sixth Cytidine can be a Thymidine. The variations inSEQ ID NO:12 which yield other oligonucleotides are that the seventhCytidine can be a Guanidine.

The procedure we have employed to amplify the V_(H) and V_(L) sequencesfrom single B cells is a "semi-nested PCR" methodology. The procedurecalls for two round of PCR amplification. The same sets of degenerate 5'V_(H) and V_(K) primers are used for both rounds of PCR, whereas the 3'primers used in the second round of PCR are derived from the internalregion toward the 3'-end of the DNA fragments amplified in the firstround of PCR. Thus, in the second round of PCR, the 5' end is notnested, while the 3'-end can be considered nested. The procedure ishence a semi-nested PCR. The 3' primers that have been used successfullyin our laboratory for V_(H) and V_(L) amplification are SEQ ID NO:13(corresponding to amino acid residue numbers 136-148), SEQ ID NO:14(corresponding to amino acid residue numbers 122-127), SEQ ID NO: 15(corresponding to amino acid residue numbers 125-135), and SEQ ID NO:16(corresponding to amino acid residue numbers 117-125).

The most appropriate number of reaction cycle in each round of PCR is 20cycles for first round and 40 cycles for second round of PCR. Theprocedure has been shown to be applicable in ARH-77, VA165, VA200, and4.8D cells. In each case, only 1/3 of cDNA synthesized from the RNAprepared from a single cell by RNA-zol extraction is required foramplification of either V_(H) or V_(K) sequence.

The V_(H) and V_(L) gene segments obtained by PCR amplification can becloned and sequenced by incorporating them into an appropriateexpression vector. For speedily screening the antibody fragments forantigen binding properties, a transient host cell expression system ispreferably employed. Supernatants taken from the transiently expressinghost cells can be used to assay the antigen-binding specificity of theV_(H) /V_(L) fragments, and their relative affinity for the antigen ofinterest.

B. Methods for selecting and isolating low-frequency antigen-specific Bcells

When the target B cell occur at a frequency of 1×10⁻⁵ or lower among theIgG-expressing B cells, and if one starts with purified IgG-bearing Bcells and if the method of selection has 1% non-specific selection, thespecific B cells among those selected will account for fewer than 1 outof 1000. Such a low frequency of specific B cells is not practical forconducting single cell-PCR, as too few antigen-specific B cells arepresent in the selected single B cells than can be processed, as apractical matter, by the single cell PCR procedure.

In the single cell PCR procedure, the V_(H) and V_(L) gene segments ofeach of the single B cells selected are to be amplified, cloned,sequenced, co-expressed, and the antibody fragments characterized.Because of the time required for analyzing each of the individual Bcells, a single cell PCR experiment can screen only about 10-20 B cells.Thus, when using PCR to isolate the gene expressing an antibody fragmentfrom single B cells, the B cells expressing the antibody of interestshould preferably account for 10% or more of the total number of B cellsprocessed. This means that the non-specific B cells included in theselection procedure must be in the order of about 1×10⁻⁴ to (0.01%) orlower, so that the antigen-specific B cells account for about 1 out of10 B cells selected and processed. This low level of non-specificity canbe achieved by the cell sorting methods of the present invention.

Certain models of fluorescence-activated cell sorters, such as the FACSStar Plus™ of the Becton-Dickinson Company, are equipped with mechanismsfor deflecting (sorting) single cells into individual containers, suchas the wells of an 8×12 (96 well) microculture plate. However, thetypical cell staining procedure employed in a cell sorting experimentwill not work for sorting low-frequency antigen-specific B cells. Forexample, for labeling B cells expressing antibodies specific for ahapten, a typical staining approach would be to conjugate the hapten toa carrier protein which is labeled with biotin and FITC-avidin. Thelabeled B cells are then stained with hapten-carrier protein-biotin andFITC-avidin. If the B cells with the desired antigen specificity occurat low frequencies, such as about 1×10⁻⁵ among the IgG-expressing Bcells, the B cells producing antibodies to the carrier protein, biotin,avidin, FITC or the cross-linkers used in conjugation will all showfluorescence, and there may be many more of these B cells than theantigen-specific B cells of interest.

In the present invention, the staining method employs two antigen probeswith different fluorochromes linked to the antigens, preferably withdifferent carriers and cross-linkers. For example, for labeling B cellsexpressing antibodies with a specificity to peptide A, the cells arelabeled with peptide A-carrier protein P1-fluorochrome F1 and withpeptide A-carrier protein P2-fluorochrome F2. The B cells that bind topeptide A, therefore, will display two colors which can be viewed in twoof the FACS channels. In contrast, the B cells binding to P1, P2, F1, orF2 will display only one color, and can be viewed in only one of theFACS channels.

To solve the background noise problem which arises due to the fact thatcertain activated cells are autofluorescent or have stickyplasmamembranes, one designs one of the fluorescence channels to gateout such autofluorescent or sticky cells. This process is referred to asnegative selection, and can be performed by one of two methods. In onemethod, the cells exhibiting positive fluoroscence for a selectedfluorochrome, such as F3, are excluded in the sorting, even though noprobe is introduced. This negative selection procedure involveseffectively sacrificing one of the fluorescence channels, i.e., the onewhich monitors F3, and selects out the cells which autofluorescence.This method has been used to reduce background noise to below 1×10⁻⁵ inan FACS sorting procedure attempting to sort single allogeneic T cellsfrom the spleens of mice, transplanted with allogenic T cells. Chang,J.-F. et al., J. Immunol. 147:851 (1991).

In another method, which can be combined with the first method, themixture of cells are reacted with an F3 conjugated probe specific for asurface marker on an irrelevant population, such as CD3 on T cells. Thecells exhibiting fluorescence for the F3 detector, either due toautofluorescence, stickiness, or positive specific reaction to the F3probe, will be excluded from sorting.

For increasing the specificity of sorting, additional fluorescent labelshave been used on the B cells to be selected. These additional reagentslabel the surface γ chain, κ or λ chain, CD19, IgG, Ia, and Fc receptorswhich are expressed by B cells. By using fluorochromes that excite atdifferent wavelengths, and by using one or two lasers, cells labeledwith two, three, or four colors can be sorted.

Because the surface antigens γ chain, κ or λ chain, CD19, Ia, and Fcreceptors are expressed by all B cells expressing IgG, the labeling ofthese additional surface molecules can help clear the nonspecificcontamination of T cells, macrophages/monocytes, B cells expressing IgM,and other cells. However, these additional labels cannot enhance to anylarge extent the frequency of B cells expressing antibody to the desiredantigen. The staining method of this invention includes the combinationof the positive selection using multiple antigen probes that containdifferent fluorochromes and do not share the same carrier molecules andcross-linkers, and the negative selection which excludes cells whichexhibit autofluorescence and membrane stickiness by gating out cellsshowing fluorescence in one of the selected fluorochrome windows.

In the present invention, the low frequency antigen-specific B cells arestained by reacting their antigen-specific receptors with two antigenprobes, both comprising the specific antigen conjugated to one of twodifferent protein carriers which are each labeled with a differentfluorochrome. Different cross-linkers also preferably used in each ofthe two antigen probes. Thus, the B cells that display two colors arenot likely to bind to either of the carrier proteins, either of thefluorochromes, or either of the cross-linkers.

It is preferred if, in addition, these B cells are labeled with a fourthfluorochrome (F4) which stains surface γ chain, CD19, or surface κchain, λ chain, Ia, or Fc receptors. With this additional label, thecells expressing three colors (for the positive selection) will mostlikely be the B cells expressing IgG specific to the antigen ofinterest.

C. Characterizing the relative antigen-binding affinities of antibodieson single cells

The relative antigen-binding affinity of antibodies on B cells can bedetermined from the relative ratio of antigen labeling intensity to IgGlabeling intensity. In the B cell antibody repertoire, theoretically anumber of clones deriving from different germ line V_(H) and V_(L) andfrom different extents of somatic mutation can bind to the sameantigenic molecule or even to the same antigenic epitope. When thenumber of sorted antigen-specific single B cells is more than what thesingle cell-PCR procedure can handle, those antigen-specific B cellsexpressing antibodies with higher affinity for the antigen should beselected for the single cell-PCR procedure, and the lower affinity onesshould be rejected.

The relative antigen-binding affinity is determined from the relativeextent of binding of antigen to the cells and of a targeting moleculewhich selects for IgG-producing B cells (preferably, an anti-IgG probefor surface IgG). To perform this calculation, a sub-job can be writteninto the computer program that processes the analysis and sorting duringthe FACS procedure. This sub-job will determine and express theintensity of fluorescence on each single cells being sorted in one oftwo different novel formats.

In the conventional format currently in use, the single cells beingsorted are assigned identification numbers. For example, if cells aresorted into wells of an 8×12 (96 well) microculture plate, these singlecells can be identified as A1, A1,A3 . . . A12, B1,B2, B3 . . . B12 H1,H2, H3 . . . H12. In one novel modified sub-job format suitable for usewith the methods of the invention, the intensity (channels) offluorescence on each single cell being sorted is compiled into a table,based on the intensity of fluorescence. If fluorochrome #1 is used forlabeling antigen, and fluorochrome #4 for labeling IgG, the ratio offluorochrome #1 to fluorochrome #4 is also expressed. The table can thenbe used to determine which cells in which wells are best suited for PCRanalysis.

In another format suitable for use with the invention, the individualsingle B cells (from the single wells) are plotted on a graph in which,for example, one axis represents the intensity of fluorephore #1 and theother axis represents the intensity of fluorephore #4. The individual Bcells which have the highest level of intensity of fluorephore #1, andthe lowest intensity level of fluorephore #4, have the highest antigenaffinity, and are most suitable for PCR analysis and expression of theirFv fragments.

EXAMPLE Selecting single B cells specific for CD4 binding site on HIV-1gp120 from infected individuals

The murine monoclonal antibody G3-519, which recognizes a linear peptideepitope in the CD4-binding site of gp120 of HIV-1, has previously beendisclosed. Sun, N.C. et al., J. Virol. 63:3579 (1989) see alsoInternational application No. PCT/US90/07535; U.S. application Ser. No.07/531,789 (incorporated by reference). G3-519 reacts with abroad-spectrum of HIV-1 laboratory strains and clinical isolates. Innumerous fusion experiments using spleen cells of mice immunized withgp120, it has been found that this particular antigenic epitope ispoorly immunogenic. It is relatively much more difficult to obtainhybrids secreting antibodies to this antigenic epitope than thosesecreting antibodies to other antigenic epitopes on gp120.

As described in PCT/US90/07535 and U.S. Ser. No. 07/531,789, G3-519reacts with a synthetic peptide, I15P, corresponding to a segment in theCD4-binding domain of gp120. Using this peptide as the solid-phaseantigen in ELISA, it has been found that the sera from HIV-1 infectedindividuals contain very low titers or undetectable levels of antibodiesagainst I15P, again indicating that this particular antigenic epitope isweakly immunogenic in human beings.

It is reasonable to assume (based on the analysis set forth above) thatbecause it is difficult to prepare hybrids from lymphocytes of mice thathave been actively immunized with gp120, the frequency of theantigen-specific B cells, which produce antibody specific forCD4-binding site of gp120, probably occur at a frequency lower than1×10⁻⁵ among the IgG-bearing B cells of gp120 immunized mice. It isreasonable to extrapolate that the frequency of these B cells in HIV-1infected individuals is probably also lower than 1×10⁻⁵ among theIgG-bearing B cells.

Using the methods of the present invention, the mononuclear cellsfraction is isolated by Ficoll/Hypaque density centrifugation of theheparinized peripheral blood from the HIV-1 infected donor. The T cellsin the mononuclear cells are removed by rosetting with sheep red bloodcells and monocytes/macrophages are removed by adhering them to thesubstratum of plastic plates or to Sephadex-G10 beads. Hudson, L. andHay, F.C., Practical Immunology (2nd. Ed. Blackwell ScientificPublications, Oxford, 1980). IgM-expressing B cells are removed bypanning with a petri dish coated with affinity-purified goat IgG F(ab')₂fragment which is specific for human IgM. Mage, M.G., Current Protocolsin Immunology, Eds. Coligan, J.E. et al. (Wiley Interscience, New York1991) § 3.5.1. The remaining cells are enriched IgG-bearing B cells,which are still contaminated with some T cells, monocytes/macrophages,IgM-bearing B cells, and numerous other kinds of white cells.

A similar preparation of cells should also be prepared from a normaldonor. These cells are to be reacted with the staining reagents, andused to set the gating thresholds for the FACS sorting.

Two fluorescence-labeled antigen probes are to be used. Peptide I15P isconjugated to two different fluorochromes. Alternatively, the peptide isconjugated to two carrier proteins, or molecules, which are each labeledwith a different fluorochrome. The preferred antigen probes are:

I15P-FITC or I15P--ovalbumin--FITC

I15-TR (Texas Red) or I15P--dextran--TR

Peptide I15P is an oligopeptide with 15 amino acid residues, none ofwhich are cysteine residues. For the purposes of conjugation, a cysteineresidue is added to the N-terminal or C-terminal end of I15P during thepeptide synthesis, preferably using a typical automated peptidesynthesizer, such as one from Applied Biosystems. The added cysteineresidues provided the free SH groups at the termini of the peptides forcross-linking to the fluorochromes or to the carrier molecules. One mayalso add a few residues, such as glycine and serine residues, betweenthe cysteine residue and the antigenic I15P peptides. The spacerminimizes stearic constraints introduced by the conjugation to thefluorochrome or the carrier protein.

The carrier molecules need not be proteins. However, proteins fromvarious organisms probably provide a large number of possible carriermolecules. The preferred carrier molecules are those which are likelynot to be antigenic in human serum. Suitable carrier molecules for usein the present invention are ovalbumin, a protein, and dextran, apolysaccharide.

A polysaccharide must be conjugated differently from a protein.Ovalbumin has a number of amino groups that can serve as the sites forcross-linking, whereas dextran molecules need to be modified to containactive groups for cross-linking. Dextran of molecular weight of15,000-20,000 can be purchased from Sigma Chemical Co. (St. Louis, Mo.).The modification is adopted from Brunswick, M. et al. J. Immunol.140:3364 (1988). Briefly, dextran is reacted with ethylenediaminedihydrochloride (Fluka, Ronkonkoma, N.Y.) to form AECM-dextran, which isthen reacted with N-succinimidyl 3-iodoacetamido propionate (SIAP, fromAldrich Chemical Co., Milwaukee, Wis.) to form SIAP-dextran. Thismodified dextran can then coupled with the peptide with the free SHgroup to form the following product. ##STR1##

As discussed above, the cross-linkers for the peptide and ovalbumin arepreferably different. A number of cross-linkers are available (forexample, from Pierce Chemical Co., Rockford, Ill.) and are suitable foruse in the invention. The choice and design of cross-linkers can also bemade by referring to the handbook by Wong, S. S., Chemistry of ProteinConjugation and Cross-Linking (CRC Press, Boca Raton 1991). Onepreferred choice for cross-linking amino groups of ovalbumin and thesulfhydryl groups is 2,4,-dinitrophenyl-p-(6-nitrovinyl) benzoate. Thestructure of the coupled product is shown below. ##STR2##

The third fluorochrome is to be sacrificed and is for excluding theautofluorescent or sticky cells. The preferred fluorochrome detector isthe one for phycoerythrin (PE). In one embodiment, the mixture of cellsneed not be reacted with a probe labeled with PE. In the gating setting,the cells exhibiting positive fluorescence for PE are excluded (negativeselection). The preferred embodiment for the present invention in thatPE is conjugated to IgG.F(ab')₂ of a murine monoclonal antibody specificfor CD3.

    MAHCD3.F(ab').sub.2 -PE

The fourth fluorochrome, for labeling surface γ chain, is an affinitypurified goat IgG F(ab')₂ fragment specific for human IgG(γ) that islabeled with allophycocyanin (APC):

    GAHIG.F(ab').sub.2 -APC

An alternative for the fourth fluorochrome is for labeling surface CD19.The preferred reagent is the mouse monoclonal antibody IgG fragmentF(ab')₂, specific for human CD19, labeled APC:

    MAHCD19.F(ab').sub.2 -APC

Purified F(ab')₂ is used instead of whole IgG because deleting the Fcportion can avoid having the molecule bind to Fc receptors on the Bcells or macrophages/monocytes. F(ab')₂ preparations of goat-anti-humanIgG and murine monoclonal antibody against human CD19 are available fromvarious commercial sources. For example, goat IgG F(ab')₂ anti-human IgGmay be purchased from Chemicon International (Temecula, Calif.) andmouse monoclonal antibody IgG F(ab')₂ anti-human CD19 may be purchasedfrom Tago, Inc. (Burlingame, Calif.). Various fluorochromes that arealready modified in various ways for conjugation are also availablecommercially, e.g. from Biomeda Corp. (Foster City, Calif.) andMolecular Probes, Inc. (Eugene, Oreg.). The modified fluorochromes andproperly modified antigens, such as I15P peptide, can be coupled to theactivated carrier molecules at the same time.

The cell preparation enriched for B cells is incubated for 30 minutes at4° C. with the mixture of the four probes directly conjugated with thefluorochromes. The incubation is performed in basic culture mediumcontaining 1% fetal calf serum. All incubation and washing fluids areculture medium containing 1% fetal calf serum (to keep the cellshealthy) to minimize nonspecific sticking of the protein probes tocells, and all procedures are performed at 4° C. to prevent capping andpatching of the bound surface antigens.

The FACS sorting procedure is performed with a FACS Star Plus™ (BectonDickinson) which has two lasers and is outfitted with an automatedcloning accessory and can sort single cells into the wells of an 8×12microculture plates. The setting of thresholds or windows for theforward light scattering, right-angle light scattering, and the fourcolors is performed with the cells from a normal donor. These cells areprepared and stained as with the experimental sample. The forward lightscattering, which can distinguish the small-sized dead cells, and theright-angle light scattering, which can distinguish the high granularityof macrophages and granulocytes, are included in the gating to cut downnoises from dead cells and granulocytes.

It should be understood that the examples, terms and expressions usedherein are exemplary only and not limiting, and that the scope of theinvention is defined only by the claims which follow, and includes allequivalents of the subject matter of the claims.

    __________________________________________________________________________    SEQUENCE LISTING                                                              (1) GENERAL INFORMATION:                                                      (iii) NUMBER OF SEQUENCES: 16                                                 (2) INFORMATION FOR SEQ ID NO:1:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                             (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                       GAGGTGCAGCTGGTG15                                                              (2) INFORMATION FOR SEQ ID NO:2:                                             (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                             (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                       GAGATGCAACTGGTG15                                                             (2) INFORMATION FOR SEQ ID NO:3:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: Single stranded                                            (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                       GAGGTACATTTGGTG15                                                             (2) INFORMATION FOR SEQ ID NO:4:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                             (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                       CAGGTGCAACTACAG15                                                             (2) INFORMATION FOR SEQ ID NO:5:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                             (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                       CAGATCAACTTAAAG15                                                             (2) INFORMATION FOR SEQ ID NO:6:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: Single stranded                                            (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:                                       CAGGTGCAGCTGCCG15                                                             (2) INFORMATION FOR SEQ ID NO:7:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                             (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:                                       GACATCCAGATGACC1 5                                                            (2) INFORMATION FOR SEQ ID NO:8:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                             (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:                                       GATATTGTGATGACT15                                                             (2) INFORMATION FOR SEQ ID NO:9:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: Single stranded                                            (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:                                       GACATCGTGATGACC15                                                             (2) INFORMATION FOR SEQ ID NO:10:                                             (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                             (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10:                                      GAAATTGTATTGACA 15                                                            (2) INFORMATION FOR SEQ ID NO:11:                                             (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                             (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:11:                                      GAAATTGTGTTGACG15                                                             (2) INFORMATION FOR SEQ ID NO:12:                                             (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 15 nucleotides                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: Single stranded                                            (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:12:                                      GAAATACTGATGACG15                                                             (2) INFORMATION FOR SEQ ID NO:13:                                             (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 29 nucleotides                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                             (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:13:                                      GAAGTAGTCCTTG ACCAGGCAGCCCAGGG29                                              (2) INFORMATION FOR SEQ ID NO:14:                                             (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 27 nucleotides                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                             (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:14:                                      CCAAGCTTGGAGCAGGGCGCCAGGGGG27                                                 (2) INFORMATION FOR SEQ ID NO:15:                                             (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 22 nucleotides                                                   (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                             (D) TOPOLOGY: Linear                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:15:                                      GGCAGTTCCAGATTTCAACTGC22                                                      (2) INFORMATION FOR SEQ ID NO:16:                                             (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 26 nucleotides                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: Single stranded                                              (D) TOPOLOGY: Linear                                                         (xi) SEQUENCE DESCRIPTION: SEQ ID NO:16:                                      CCAAGCTTCATCAGATGGCGGGAGAT26                                              

I claim:
 1. A method of selecting antigen-specific B lymphocytes from a mixture of cells using a fluorescence activated cell sorting apparatus which has several channels, each of said channels capable of viewing a different fluorochrome, comprising:labeling B lymphocytes in the mixture with at least two different antigen probes, wherein each probe comprises an antigen which binds the antibodies on the target B lymphocyte surface, said antigen conjugated with a fluorochrome, wherein said fluorochromes yield different colors upon activation; selecting B lymphocytes which are positive for both fluorochromes attached to the antigen probes.
 2. The method of claim 1 wherein one antigen probe is labeled with FITC and the other antigen probe is labeled with Texas red.
 3. The method of claim 1 wherein three probes are used, each associated with a different fluorochrome, and further including labeling B lymphocytes with a targeting molecule specific for a marker unique to B lymphocytes, and wherein the targeting molecule is labeled with a fluorochrome yielding a color different from those of the antigen probes.
 4. The method of claim 3 wherein the targeting molecule is F(ab')₂ specific for IgG.
 5. The method of claim 3 wherein the marker is the surface IgG, κ and λ chains, CD19, Ia, or Fc receptors which are expressed by B cells.
 6. The method of claim 3 wherein the fluorochrome which the targeting molecule is labeled with is allophycocyanin.
 7. The method of claim 3 wherein, in the antigen probes, a carrier molecule is conjugated between the antigens and the fluorochromes.
 8. The method of claim 7 wherein a different carrier molecule is associated with each antigen probe.
 9. The method of claim 8 wherein one carrier molecule is ovalbumin and the other is dextran.
 10. The method of claim 9 wherein the quantity of fluorochrome on each individual B lymphocytes is determined from the intensity of color from the fluorochrome, and this value is used to determine which B lymphocytes should be separated from the B lymphocytes population.
 11. The method of claim 10 wherein for each B lymphocyte, the ratio of the quantity of fluorochrome from one of the antigen probes over the quantity of fluorochrome from the B lymphocyte marker is determined.
 12. The method of claim 11 wherein the B lymphocytes with the highest of said ratio are determined to have the highest affinity for the antigen.
 13. The method of claim 12 further including amplifying by PCR the V_(H) and V_(L) gene segments of the labeled B lymphocytes which were sorted from the B lymphocyte population. 